Removable coupling devices such as hooks are commonly used to temporarily connect two separate objects. Often, it is necessary or desirable to provide a removable connection that can be locked in order to ensure that the two objects are not inadvertently separated before it is desirable to do so. For example, three-point hitch systems for tractors and other agricultural vehicles often feature an upper link having a hook that operates in a locking fashion to prevent detachment of an implement from the vehicle during its use. While described in this context, the present invention is not intended to be limited to this particular application.
Upper hooks for three-point tractor hitch systems that lock a coupling sphere or pin of an implement within the mouth of the hook are currently available to prevent accidental separation of the tractor and implement during use. U.S. Pat. No. 4,241,935 and European Patent Application Publication No. 1,403,101 both describe such upper hooks having a latch for controlling the motion of a locking bolt passing through an opening in the hook body for selectively blocking a portion of the hook mouth housing a coupling sphere within. The inventions in the above patent documents each provide a coupling eye on the control lever for attachment of a cable by means of which the control lever can be actuated between latching and unlatching positions from a distance. While this feature allows connection of the upper hook to the coupling sphere without the user having to dismount the tractor seat, actuation of the locking components by the user is still required.
U.S. Pat. No. 6,478,094 describes another three-point tractor hitch which allows the attachment of an implement without requiring the operator to leave the tractor seat. The upper link of the hitch includes a stationary jaw having a movable jaw pivotally attached to it in such a way that lowering the movable jaw onto a coupling pin of the implement will cause the movable jaw to pivot upward into the stationary jaw. In this position, the coupling pin is held within the two jaws and a latching mechanism prevents further relative motion of the jaws, thereby locking the pin in place. The length of the upper link is adjustable from the tractor seat as the jaws are connected to the tractor by a linear hydraulic actuator. This invention does not require a control lever to be held in an unlatched position during connection of the hitch and implement. However, the invention requires accurate placement of the upper link such that the movable jaw is positioned directly over the coupling pin, which despite the addition of the hydraulic actuator that can be operated from the tractor seat, can still be difficult and time consuming for a single user.